Producers of colored products and dispersions seek to achieve a consistent color of their products using many techniques. One simple method involves halting a pigmentation process, applying a coat of the material to a surface and comparing the dried coat to a standard. This method is imprecise and susceptible to unpredictable or untimely production.
Other methods involve evaluating a liquid during the pigmentation process. For example, one method includes directing a portion of a dispersion through a flow cell. One technique provides for observing an illuminated dispersion flowing through the flow cell for comparison with a standard. Another technique directs light through the dispersion in the flow cell onto a spectrometer which compares the characteristics of the light received with specified characteristics.
The known methods are inappropriate for use in connection with production of pearlescent or “effect” interference pigments. Such pigments based on platy substrates which have been coated with a metal oxide layer are well known. These pigments exhibit pearl-like luster as a result of reflection and refraction of light. Depending on the thickness of the metal oxide layer, they can also exhibit interference color effects.
Commercially, the pearlescent pigments encountered most often are the titanium dioxide-coated mica pearlescent pigments and the iron oxide-coated mica pearlescent pigments. They are made by forming a hydrous layer(s) on the substrate and then calcining the composite. The color of the hydrous and calcined layers need not be identical. The color generated is a function of the optical thickness of the coating, which in turn is a function of the refractive index of the coating and the physical thickness of the coating. The physical thickness is a function of the coating process and its parameters and conditions.
Control of the product characteristics is complicated by the fact that free particles of the coating, unattached to a platy substrate, can form and effect the apparent color and also because the color changes rapidly as a result of high reaction rates. To use known methods of color monitoring, therefore, the manufacturing process must be halted, a sample of the in-process material obtained and dried (and also calcined if the end product is intended to be calcined), and then the resulting color characteristics must be compared to a standard. This is clearly not optimal.
To accurately monitor color, it would be necessary to obtain and dry a sample of the pigment, suspend it in a coating carrier and coat a color evaluation substrate before evaluating the color. This also is clearly impractical and time consuming. Typically, therefore, interference effect pigment processing involves a simple subjective visual observation of pigment dispersion as the hydrous coating is being formed on the substrate and maintaining the processing conditions as close to predetermined parameters as practical.
What is needed is an apparatus and a method for objectively ascertaining a color match between a selected standard color and the color exhibited by an interference effect pigment dispersion during processing thereof to allow terminating the process upon achieving a match.